A CO2 gas-discharge slab-laser comprises an unstable resonator constrained in one transverse axis by a waveguide formed between spaced-apart, parallel, planar discharge electrodes of the laser. Typically the electrodes are spaced apart such that plural spatial lasing-modes are not supported in that transverse axis, which is perpendicular to the plane of the electrodes.
In a plane parallel to the plane of the electrodes, a plurality of spatial lasing-modes is generated by the unstable resonator. Because of this an output-beam from the laser, in the far field, absent any measures to control the beam, comprises a central mode (lobe) with increasingly weaker lobes (side-lobes) on opposite sides of the central mode, with all lobes aligned with each other in a direction parallel to the electrode-plane.
CO2 slab-lasers are used extensively for laser-drilling operations in general, and for drilling via-holes in printed circuit boards in particular. A single spatial-mode beam is necessary for such drilling operations, preferably with a circularly symmetrical intensity distribution, most preferably in Gaussian form.
In order to provide such a single spatial-mode beam, adjustable parallel knife-edges are used to block passage of all side-lobes of the beam, passing only the central mode. Beam shaping optical elements are used to make the beam cross-section rotationally symmetrical, and a collimating lens is used to collimate the beam for transmission to focusing optics of remotely-located drilling optics.
It has been observed in certain instances, that if the collimated beam is transmitted a distance greater than about 3 meters (m) from the laser, the beam cross-section (beam-quality) can become progressively degraded with increasing transmission distance, thereby adversely affecting quality of holes drilled by the beam. There is a need for improving the quality of CO2 slab-laser beams transmitted over distances significantly greater than 3 meters.